oob -> urn:ietf:wg:oauth:2.0:00b.

Reviewed in http://codereview.appspot.com/5647045/.
diff --git a/oauth2client/tools.py b/oauth2client/tools.py
index 236c859..eeb9031 100644
--- a/oauth2client/tools.py
+++ b/oauth2client/tools.py
@@ -30,6 +30,7 @@
 import webbrowser
 
 from client import FlowExchangeError
+from client import OOB_CALLBACK_URN
 
 try:
   from urlparse import parse_qsl
@@ -120,7 +121,7 @@
   if FLAGS.auth_local_webserver:
     oauth_callback = 'http://%s:%s/' % (FLAGS.auth_host_name, port_number)
   else:
-    oauth_callback = 'oob'
+    oauth_callback = OOB_CALLBACK_URN
   authorize_url = flow.step1_get_authorize_url(oauth_callback)
 
   if FLAGS.auth_local_webserver: